| This dissertation presents the first observations of self-assembled arrays of epitaxial nano islands in ceramic systems, based on RF sputtering and thermal processing of Gadolinia-doped ceria (GDC) thin films on an yttria-stabilized zirconia (YSZ) single crystal substrate. In contrast to the conventional semiconductor nano island self-assembly systems, the island arrays in the GDC-YSZ system provide materials with categorically different physical properties and functionalities, and they exhibit a stronger ordering at a larger characteristic length scale.; The initial focus of this study was on the processing and characterization of thin GDC layers on YSZ, which are used in SOFCs as barriers to prevent the reaction of some cathode materials with the YSZ electrolyte. Chapter 3 of this document describes studies on relatively thin (<200 nm) GDC deposits which remained adherent to their substrates during post-deposition processing. The GDC films were amorphous or ultra-fine grained as deposited, with a mixed GDC-YSZ layer at the interface. After annealing at 1150°C, the GDC films were epitaxially oriented on the YSZ substrates, with isolated porosity in their interior.; Some of the thick RF-sputtered GDC layers (>300 nm) were found to fail by spalling from the YSZ substrate, leaving behind patches of unspalled film and exposing a sputter-mixed GDC-YSZ surface. Upon annealing, the modified surface spontaneously broke up into two-dimensional arrays of epitaxial islands with sub-micron dimensions, exhibiting order in spacing and alignment.; In addition to the classical local effects that drive dewetting processes, the self-assembly of the epitaxial GDC-bearing islands is driven by elastic interactions between them, and these interactions are mediated by the elastically anisotropic underlying YSZ substrate. The stresses in the initial mixed surface layers are modified by two factors: The thermal-expansion mismatch leads to stresses, depending on temperature and heating rates. The lattice-parameter mismatch leads to coherency stresses, depending on chemical composition variations and interdiffusion rates. When the initial surface layers cannot relieve their stress by other mechanisms, they break up by surface diffusion to introduce stress-free surfaces and relax the stresses in their vicinity, at the expense of increasing surface area and energy. The balance of surface and elastic energy leads to islands of characteristic size, arrayed periodically and aligned in directions of low elastic stiffness in the underlying substrate. After extensive annealing (e.g. ∼80 hrs at 1150°C), the predominant source of stress is the non-uniformity of composition, which is reduced by bulk interdiffusion, ultimately leading to the dissolution of the islands into the YSZ substrate. The transient nature of the island formation process was confirmed experimentally, and it points to the need for tuning processing variables to find conditions where the rate of surface diffusion is significant, but processes of bulk interdiffusion and misfit dislocation formation are relatively sluggish.; A first order calculation indicated that less than 1 GPa of stress was needed to drive the observed morphological changes in the GDC-YSZ system. Any of the three sources already mentioned (misfit, thermal stress, or residual from the deposition) provided sufficient stress to cause spontaneous breakup. The relative contribution of each source was unknown, but there is no doubt that elastic interactions were dominant in guiding reordering. Electron diffraction showed the islands to have a simple cube-on-cube orientation relationship with the substrate and they were aligned in the <110> directions on the (001) YSZ surface. This was the most elastically compliant direction available on the (001) surface with the <110> directions being 99.5% more compliant than <100>. Also, the islands were highly size similar, providing further evidence that elastic inter... |
